Oil consumers and producers united on Friday to warn that extreme price volatility was dangerous for the world economy, but disagreed on what an appropriate price should be.

The disagreement in a meeting in London came as oil prices tumbled more than $3.50 to below $34 a barrel, a four-year low, amid concerns about the global economy and Opec’s ability to cut its production.by the 2.2m barrels a day it promised earlier this week.